The continuous rise in the world population and the expansion of industrial facilities around the globe has placed a growing demand on the fresh water supply from natural resources, such as rivers, fresh water lakes, underground aquifers, and brackish wells. These resources have been steadily on the decline since the early 1950's. Therefore, there is clearly a need for new fresh water resources to balance the growing consumption rate.
Since 96% of the earth's surface is covered with saline water, there has been and continues to be strong motivation for developing water desalination technologies. Today there are more than 7,500 desalination plants in operation worldwide, and about two thirds of those are operating in the Middle East. Saudi Arabia operates the largest desalination plant, with a capacity of 128 MGD. The United States accounts for about 12% of the world's desalination capacity.
Desalination involves any process in which dissolved minerals are removed from saline or brackish water. Technologies for desalination include distillation, reverse osmosis, electro-dialysis, and vacuum freezing. Distillation and reverse osmosis are the most common. Distillation technologies include Multiple Effect Distillation (MED) and Multi-Stage Flashing (MSF), both of which operate by evaporating saline water at atmospheric or reduced pressure and condensing the vapor to produce fresh potable water. Reverse Osmosis (RO) operates on a filtering principle. High pressure pumps force saline water through nanofilter membranes allowing fresh water to pass while filtering out the dissolved minerals. Although distillation and reverse osmosis technologies currently provide the most cost effective method for desalination, they are both very energy intensive. Accordingly, whether or not these desalination techniques remain cost effective strongly depends on energy prices.
A desalination technology that has drawn interest over the past two decades is referred to as Humidification Dehumidification (HDH). According to the HDH process, saline water is pumped through a condenser coil, where it picks up heat from condensing water vapor. The saline water is then pumped through a solar collector where it collects more heat. The saline water is then sprayed in a cooling tower, where a portion of it evaporates into the air. The water vapor is then condensed over the condenser coil of a conventional tube condenser. An advantage of this type of technology is that it permits low pressure, low temperature desalination. El-Bourouni et al. (2001), El-Hallaj et al. (1998), and Assouad et al. (1988) respectively reported the operation of HDH units in Tunisia, Jordan, and Egypt, respectively.
Another type of desalination technology that makes use of water evaporating into an air stream is the Carrier-Gas Process (CGP) reported by Larson et al. (1989). A CGP system consists of two chambers separated by a common heat transfer wall. One chamber is used for evaporation and the other for condensation. Ambient air is directed through the evaporation chamber and mixed with high salinity feed water. The air picks up heat from the heat transfer wall and increases in temperature as it moves through the evaporation chamber. Some of the feed water evaporates into the air and the rest is removed as concentrate. The humidified air is heated in a heater and is then sent to a condensation chamber where the water vapor is condensed out. Purified water is collected in the condensation chamber.
Even though both HDH and CGP may provide some level of improved efficiency compared to more conventional desalination methods, HDH and CGP are both still energy intensive and provide limited water production efficiency (e.g. kilograms of fresh water per kilogram of feed water). Accordingly, even these improved methods still generally have limited application.